Method of polymerizing unsaturated methylene compounds



Feb. 13, 1945. C BARNES 2,369,520

METHOD OF POLYMERIZING UNSA'IURATED METHYLENE COMPOUNDS Filed Oct. 6, 1939 2 Sheets-Sheet l SHAH/VIM [FY/Q1. EEHR/VES Feb. 13, 1945. c. E. BARNES METHOD OF POLYMERIZING UNSATURATED METHYLENE COMPOUNDS 2 Sheets-Sheet 2 Filed Oct. 6, 1939 w 5 2 m n m 7 4 o E (E a 4? L 5 4 4 n 5 a a 5 4 5 3 w 7 mm |!\\\\|I H .l 2 Z 2 w Patented Feb. 13, 1945 METHOD OF POLYMEBIZING UNSATURATED METHYLENE COMPOUNDS Carl E. Barnes, Worcester, Mass, asslgnor, by

mesne assignments, to E. I. du Pont de Nemours dz Company, Wilmington, Del., a corporation oi. Delaware Application October 8, 1939, Serial No. 208,178

I Claims.

The invention relates to a method of polymerizing unsaturated methylene compounds:

One object of the invention is to provide tor uniformity in the polymerization of unsaturated methylene compounds. Another object of the invention is to provide a method for polymerizing such compounds which will eliminate the possibility of discoloration. Another object of the invention is to insure the production or clear, colorless, transparent polymers of uniform characteristics. Another object of the invention is to permit mass production of polymeric products with a minimum of second grade material. Another object of the invention is to speed up the polymerizing process and in general to speed up production. Another object of the invention is to provide a method 01 the type indicated which is relatively simple and easily controlled. Another object of the invention is to produce polymers having diiferent physical properties, viz. higher softening point and lower solubility (as a result of higher molecular weight). Other ob- Jects will be in part obvious or in part pointed out hereinafter.

The invention accordingly consists in the several steps and relation and order of each of said steps to one or more of the others thereof, all as will be illustratively described herein, and the scope of the application of which will be indicated in the following claims.

In the accompanying drawings showing one of many possible embodiments of apparatus for practicing the method of the invention,

Figure 1 is a plan view: 7

Figure 2 is an end elevation of'the apparatus;

Figure 3 is a sectional view taken on the line 33 of Figure 2;

Figure 4 is an enlarged sectional view taken on the line l4 of Figure 3;

Figure 5 is a side view of an end plate;

Figure 6 is an end view 01' an end plate.

The method of the present invention is applicable to the lower alkyl esters of acrylic acid and methacrylic acid. These compounds are capable of forming peroxides by reaction with oxygen. Examples of such compounds are methyl acrylate and methyl methacrylate. The invention also applies to the interpolymerization of mixtures of these substances as, tor example, methyl methacrylate modified by the addition of methacrylic acid or methacrylic anhydride. These interpolymers are useful for optical purposes, windows, Windshields or the like and it is important for many such uses that they be colorless as well as transparent.

In carrying out the invention I either make or select the desired monomer or monomeric mixture and if it is not obtainable in a, relatively pure form I purify it. Having provided a quantity of monomeric unsaturated methylene compound or mixture of such compounds, I introduce into one or the other or both or form in one or the other or both a peroxide. So far as certain features 01' the invention are concerned, I may use benzoyl peroxide, 'acetyl peroxide or other peroxide not derived from one o! the monomeric substances. When using benzoyl peroxide in a specific example of methyl methacrylate and 20% methacrylic acid, 01% oi benzoyl peroxide will sufflce.

On the other hand, in the preferred form of my invention I form the peroxide in situ in the monomer. That is to say, I form, for example, methyl methacrylate peroxide, which is believed to have the structure:

CHr- -coocu.

This may readily be done by exposing methyl methacrylate to air or still more quickly by exposing it to oxygen. A practical way to accomplish this is to let the monomer stand in a glass container at room temperature open to the air and in the light. The application of more best, light, or an increase in the concentration of oxygen will hasten the process. A desirable concentration of the peroxide is 0.005% and this may be obtained either by stopping the reaction when this amount has formed or by diluting a more concentrated'solution with monomer containing a smaller amount of peroxide or no peroxide at all. I may keep the monomeric substance essentially peroxide-free by storing it in tightly corked container in the dark at low temperatures, such as in a dark icebox at 0 C.

I may also obtain the desired concentration of peroxide by adding the concentrated oily peroxide in the correct proportions to the monomeric substance. This peroxide may be prepared by evaporating a solution of the peroxide which has been formed-in the monomer under reduced pressure to minimize polymerization. The peroxide is obtained as a viscous oil. This peroxide is believed to be the peroxide of the monomeric material having the formula (using vinyl acetate as an example):

As a means of determining the concentration of the peroxide, I employ a colorimetric test in which the yellow color of iodine liberated from a freshly prepared solution of potassium iodide in water is compared with the color of iodine liberated from a similar solution oi known peroxide content. Because of the instability of the color owing to the liberation of more iodine by the action oi light, it was found more practical to make up standards by diluting an aqueous solution of potassium dichromate to match the color of the iodine liberated from a solution oi peroxide oi known concentration. A series or such dichromate standards was made up by comparison against solutions of benzoyl peroxide in the monomer (methyl methacrylate was used) ranging in concentration from 0.0001% to 0.01%.

Th procedure followed was to add distilled water to a small amount of reagent grade potassium iodide crystals in proportions to insure a saturated solution. The liquid containing the peroxide was then added and the mixture shaken vigorously for one minute. The comparison with the standard was then made immediately. The volume oi liquid was kept constant so that equal depths were always compared.

The reaction of forming peroxides in polymerizable unsaturated methylene compounds is greatly influenced by the action of light, but at slightly elevated temperatures it can take place in the dark. The eil'ect of light alone is shown in Table I. The percentages of peroxide are given as equivalents to benzoyl peroxide.

Tut: I

Formation of peroxide in methyl methacrylate exposed to air at 27 C. in the light and in the dark [Percent peroxide-benxoyl peroxide] louwatt bulb Time, days at s i ii s isnce Dark 0001 Less than 0.0001 .0026 0001+ Peroxide formation in the dark at 50 C'. in an atmosphere of oxygen Tim i P id t eo perox esubm heating benaoyl 9 peroxide Hours Methyl metbscrylnte 82 0. 0l Vinyl acetate 32 0. 0075 Peroxide formation in some unsaturated methylene compounds is catalyzed more by light, and in others more by heat. An examination of Table III will show that light has a relatively greater eil'ect than temperature in forming peroxide with vinyl acetate, while the situation is reversed with methyl methacrylate. That the equilibrium conditions at 65 0. do not prevent the formation of more than 0.02% peroxide in vinyl acetate was shown by building up a concentration of about 0.04% by irradiation oi. vinyl acetate with ultraviolet light at room temperature and then heating at 65 C. in the presence of air for a period oi a,seo,sac

iii'teen hours in the dark. The concentration of peroxide was found to have increased slightly by this treatment.

TAIL! III Comparison of the relative eflect of heat and light on peroxide formation [Time of treatment: 17 hours in walled tubes in an atmosphere of type -i mercury arc with onto: glass removed.

In the above and other tables and formulae the symbol UVL signifies ultra-violet light.

It is thus established that, in the presence of oxygen, unsaturated methylene compounds form peroxides, and do so under the same conditions that induce polymerization. This is highly signiflcant tor it indicates that the mechanism of peroxide formation is intimately connected with that of polymerization.

The relationship between peroxide iormation and polymerization under the influence or light may be postulated as follows:

(1) M-l-UVIHM' (2) (a) M'+Oa-NO:

(b) M'+M- MM' Where M =monomeric unsaturated methylene com- M'=activated monomer molecule, MM =polymer chain, and MO: =peroxide If it is postulated that in the two competing reactions of step 2 the reaction of M with oxygen takes place more readily, and when oxygen is present, to the exclusion of b, then the inhibiting action oi oxygen in the photopolymerization oi vinyl acetate can be easily understood. The aechanism in the absence of oxygen would then If, however, this is the true explanation of the inhibiting action or oxygen in the photopolymerization of unsaturated methylene compounds, oxygen should likewise exert an inhibiting action on the thermal polymerization of these compounds. This assumes, of course, that a similar mechanism, also involving the formation of M, applies to heat polymerization as well.

This is not in accord with frequent observations that oxygen is a catalyst for the thermal polymerization 0! many unsaturated methylene compounds. But the observed catalytic effect of oxygen in the thermal polymerization of unsaturated methylene compounds has never been separated from the catalytic eii'ect or peroxides.

It has been demonstrated (Tables II and III) that contact with oxygen in the dark at elevated temperatures results in the formation of peroxide. That it is the peroxide and not the oxygen which is the catalyst is shown by the experiment summarized in Table IV. This experiment furaeeasao ther shows that oxygen is definitely an inhibitor in the thermal polymerization of methyl methacrylate as well as in the photopciymerization.

I have thus discovered the surprising fact that oxygen has a. very important effect on the heat polymerization of these compounds. If the amount of oxygen in contact with the unsaturated methylene compound, or mixture of unsaturated methylene compounds, is carefully con trolled, superior products are obtained.

TABLE IV Oxygen as an inhibitor in the thermal polymerization of methyl methacrfllote. Sealed tubes were heated at 65 C. in the dark for 4% hours Catalyst Atmosphere (benwyl Viscosity peroxide) P" 61 S lid 1d til N 0i 0. 0 .won no ow.

. 0.01 Viscous, but readily flowed,

No polymer, but equivalent of 0.0025% bcnzoyl peroxide.

None

will take place in preference to the reaction,

M +M MM* This has been demonstrated by heating a sample of methyl methacrylate at 65" C. in the dark in contact with a constant pressure of oxygen for a period of twenty-six hours. During this time, it absorbed oxygen at a steadily increasing rate. At the end of the heating it had absorbed twice its volume of oxygen (at 65 0.), and had not visibly increased in viscosity (i. e., less than l% polymer). A portion of this sample, after the oxygen absorption, was placed in a tube under nitrogen and a similar portion under oxygen, and heated in a bath at 65 C. The portion under nitrogen polymerized to a solid in a few hours, but no visible change took place in the sample under oxygen.

In a sealed tube at elevated temperatures the oxygen is soon removed as peroxide and then polymerization proceeds rapidly in an inert atmosphere, catalyzed by the peroxide formed. This was demonstrated by heating a tube containing methyl methacrylate under one atmosphere of oxygen for fifteen hours at 65 C., and then opening under water. During this time the substance had polymerized to a solid, and upon opening the tube approximately 80% of the free volume of the tube was filled with water. The unfilled volume was probably due to some methyl methacrylate vapor, since the tube was still warm.

In a sealed tube with air present the polymerization rate will appear autocatalytic in nature since during the first stages the reaction will be one of peroxide formation. As the oxygen becomes converted to peroxide, its inhibiting eifect is reduced and the increased concentration of peroxide results in more rapid formation of active centers. If the polymerization is carried iii] out in air, the increase in rate will be due solely to the increase in catalyst concentration. Some compounds, such as vinyl acetate, are very sensitive to oxygen.

It is thus apparent that the only difference between photo and thermal polymerization is in the mechanism of the formation of the active centers. In photopolymerlzation the activated monomer molecules may be formed as a. direct result of the absorption of energy from ultraviolet light. Polymerization cannot occur at the customary temperatures in the dark unless oxygen or some catalyst is present.

At elevated temperatures, however, polymerization can occur in the absence of oxygen or catalyst. Thus, vinyl acetate will polymerize at 180 C. and methyl methacrylate at 170 C. There is undoubtedly a critical temperature for each unsaturated methylene compound below which the rate of formation of active centers (i. e., rate of polymerization) is insignificant in the absence of catalyst. An experiment with methyl methacrylate in a vacuum indicates that it may be possible for the uncatalyzed thermal polymerization to proceed at temperatures as low as 65 0., although at a comparatively slow rate. The presence of a catalyst greatly increases the rate at lower temperatures. As ordinarily carried out, this catalyst is the peroxide. Since this cannot be formed from active centers, it must be concluded that a direct addition of oxygen to the unsaturated methylene compound takes place:

The peroxide, once formed, is the catalyst for the production of the active centers.

Experiments have shown that the presence of 0.01% of benzoyl peroxide in most unsaturated methylene compounds in an inert atmosphere is sufllcient for rapid and complete polymerization at 65 C. Methyl methacrylate, for example, requires but five or six hours under these conditions to solidify. If each peroxide molecule were capable of initiating only one polymer chain, the molecular weight of the resulting polymer would be about 1,000,000, since the concentration of peroxide is such that there is about one molecule of peroxide to 10,000 molecules of methyl methacrylate of molecular weight 100. Since this value for the molecular weight is unreasonably large, it is clear that either the peroxide catalyst must be capable of regeneration to some extent, or the growing chains must be capable of producing active centers, perhaps by a chain transfer mechanism of the type suggested by Flory in 59 J. Am. Chem. Soc. 243.

Starkweather and Taylor concluded "that benzoyl peroxide is destroyed during the polymerization of vinyl acetate in solution. Cuthbertson, Gee and Rideal have found that benzoyl peroxide dissolved in toluene decomposes at 100 C. approximately, according to a first order reaction.

This suggests a mechanism whereby a small amount of the peroxide catalyst might polymerize the whole mass.

1ggiarlrweather and Taylor, J. Am. Chem. Soc. 52, 4714 gg iibertson, Gee and Bideal, Proc. Royal Soc. 110, 302

oxygen could bring about the polymerization of any quantity of monomer in time. However, experiment shows that methyl methacrylate peroxide is stable under the same conditions that promote poylmerization. A sample of methyl m thacrylate peroxide dissolved in ethyl acetate and heated to 65 0. showed no signs of decomposition after a period of seventeen hours. Inasmuch as the polymerization of methyl methacrylate in the presence of its peroxide proceeds rapidly at this temperature, the above mechanism is hardly tenable. It would also be necessary to assume the decomposition of benzoyl peroxide at this temperature, but experiment shows that benzoyl peroxide is also stable in ethyl acetate at 65 C.

These facts would also seem to exclude the possibility of the peroxide decomposing as the result of a collision, according to any of the fol- A mechanism free of these objections is one involving a complex,

When the catalyst is a "foreign" peroxide, such as benzoyl peroxide, the mechanism could be postulated as follows:

(a) (BZaOsM)- BZ20:+M' (b) (Bz:OaM) MO:+decompositlon products If the decomposition of the complex should take place according to (b), the monomer peroxide could then serve as the catalyst. The mechanism of the catalyzed reaction is still not clear.

In the foregoing mechanisms the active center has been designated as M without reference to its nature. The most reasonable assumption is that M' is a free radical. The ease with which unsaturated methylene compounds may form free radicals may be better understood by a consideration of their electronic structure.

The central pairs oi electrons in ethylene. a

balanced molecule,

are evenly distributed between the two carbon I B H H H Il:X: HzX:

I II

The energy required to bring about this change assasao etc.

There are many diii'ersnt designs possible for an apparatus to polymerize these unsaturated methylene compounds or their mixtures under these conditions. One very suitable form is shown in the accompanying drawings. Referring first to Figure l, I provide a base III which may take the form of a U shaped metal casting. It is desirable that the apparatus be sumciently massive to run with a minimum of vibration and accordingly I preferably provide a fairly heavy casting. So far as the method of the invention is concerned. the axis of the rotatable mold may be vertical or horizontal or at any other angle and so I have shown the base as U shaped whereby the apparatus may be placed in many diiierent positions but for many practical purposes a simple base consisting of a plate of iron will suffice.

Upon the base I I I mount an electric motor H which may be of any desired type. For example, I may use a 110 volt A. C. motor adapted to be controlled by a rheostat to vary the speed. This motor I I has a pulley I2.

I provide a head center I! and tail center I. The head center II is slidable in a massive journal II which is part of a headstock It that is fastened to the base II. The tail center I4 is carried by a sleeve I! which is engaged by a screw ll operated by a handle I! journalled in a journal member 20 attached to a journal portion 2| of a tailstock 22 which is fastened to the base III. By releasing a screw 23 the head center i! may be adjusted within wide limits. By tuming the hand wheel ii the tail center ll can be adjusted within precision limits. Such apparatus is well known in machine tools, for example, in lathes and grinding machines, and need not be further described herein.

Referring now to Figure 4. I provide a pair of mold halves 25. 2!. These have flanges 28 whereby they may be Joined together to form a hollow cylinder. In the present invention it is highly desirable that the internal cylindrical surface of the mold 25, 2! be as perfect as possible and that leakage be eliminated and no objectionable foreign bodies present. Accordingly I may make the mold by forming a. pair of cast ings roughly to the shape of the half mold ll. then grinding and lapping the joining surfaces 21, then after assembly of the mold by internally grinding its interior cylindrical surface 28. The surface 28 is then silvered. The mold flanges 2| may be held together by bolts I0 and nuts 8 I. To make the cylindrical mold liquid tight without inhibiting Polymerization or causing dicoloration of the comopund, I may use a gasket material Ila between the flanges 26 and the joining parts of the mold, for example a gasket of "Thiokol," which is a trade-mark for a particular brand of an ethylene dichloride sodium polysulphide reaction product.

The mold halves 28 further have at each end end flanges 35. Referring to Figure 3, I provide end plates and 31. The ends of the end flanges I! are carefully ground on a surface grinder and then lapped to a mirror finish and then silvered. The flat inner surfaces of the end plates 38 and 31 are likewise ground, lapped and sllvered. Figures and 6 illustrate the end plate 31 which has a pulley groove 38 formed in the periphery there- 01'. The end plate 31 likewise has a plurality of holes 39 which match corresponding holes in the underlying flanges 35. The end plate 31 likewise has two stop cocks 40 located in balanced position thereon. The end plate 31 also has a central hub II which has a center hole 42.

The end plate 38 is of slightly smaller diameter than the end plate 31 and has no pulley groove 38 nor stop cocks lll; otherwise it is the same as the end plate 31.

Through the matching holes 39 in one end plate It and one flange I and through the matching holes 39 in the other end plate ill and the other flange I! extend bolts 45 upon which at one end are nuts 41. These hold the end plates 30 and 81 onto the molds 2t, and gasket material oi Thiokol can be used likewise to make a gas-tight union at the juncture of the end plates with the cylindrical part of the mold.

The procedure is as follows: The mold halves 25, 25 are first clamped together by means of the bolts with the gasket of Thiokol 260 between them. One end plate, for example the end plate 86, is clamped to a flange with the Thiokol gasket ll interposed in position therebetween. A weighed amount of monomeric polymerizable compound with peroxide is now introduced into the open end oi the mold. Then the other end plate 31 is placed in position, likewise with a gasket 0! Thiokol l8 interposed and the permanent clamping bolts 45 are inserted and tightened. The air is now evacuated from the mold, using one of the stop cocks and a commercial air pump. Pumps which will evacuate to a pressure of one-tenth oi a millimeter are readily available. The entire mold is now placed between the centers l3 and II, these centers resting in the center holes 42 of the hubs II. The tail. center is adjusted, as by means oi the hand wheel N, to tighten the centers so that the mold will just turn without too much friction but without any play or vibration at all. A belt 50 is placed in the pulley groove 38 and over the pulley The entire unit is now placed in an oven or heating chamber of any desired construction and by means of an electric current the motor H is energized. Before the oven door is closed and the heat turned on, the motor II is started and its speed adjusted by means of the rheostat to give the desired number of revolutions per minute to the mold. It may be noted at this point that the mold is rotated so rapidly that the force of gravity is a minor force and centrifugal force is the controlling force. For example, if the mold axis is placed in a vertical position and if the mold is flve centimeters in diameter and ten centimeters high and rotated at a thousand revolutions per minute, the effect of gravity as measured by the thickness difference of the polymer at the top and bottom of the mold will be approximately iive thousandths of a centimeter (.005 cm). Thus the mold axis may be horizontal or vertical or at some angle between the two. When the mold axis is horizontal, the eiiect of gravity will not make a difference in the thickness of the polymer at diflerent ends of the mold but if picked up speed to turn it to a horizontal position. There are some advantages, therefore, in having an apparatus which can be placed in either a vertical or a horizontal position.

With the mold rotating in the oven, the temperature is raised to the desired point for polymerization and the apparatus is kept in the oven for the desired period oi time, whereupon it is removed from the oven and the mold is taken from the centers 13 and I4. The mold may now be opened, which is done by removing the bolts 45, then removing the end plates 36 and 31, then removing the bolts 30 and separating the two mold halves 25. Although the parts may stick together somewhat, a ready means of separation is to provide tapped holes ii in one flange 26 but not in the other and round nosed screws or bolts can then be inserted and screwed into the tapped holes which will eflectively separate the mold halves. The end plates 38 and 31 can be removed by a similar procedure, using tapped holes 52 through the end plates which have no matching holes in the underlying flanges II. The mechanical expedients can be widely varied and especially if the mold is given a slight taper it is possible in the case of certain polymers to use a single piece mold. that is, apart from the end plates. Furthermore, instead of Thiokol gaskets. silver wire gaskets may eilectively be used. I find that the most practical metal from which to make the molds is brass which is usually free from blow holes and blebs and is readily silver plated. I could use silver excepting for the expense,

After the liquid has polymerized and has been removed from the mold, it will be found to be a hollow cylinder. In the case of many polymers this hollow cylinder can be made into a sheet by cutting it along the cylindrical element, then opening it up. By heating and pressing the material, all strains due to its formation as a cylinder can be removed in the case of most polymers and from the sheet discs can be died out for use for optical purposes or to make other desired shapes. From the disks can be molded or ground optical eye pieces or other lenses.

Example I Using the apparatus of the invention, a mixtureof monomeric methyl methacrylate and 15% b weight of methacrylic acid is placed in the mold. The mixture of monomers contains of the order of .005% of peroxide calculated as benzoyl peroxide. Having placed the monomers in the mold, as already described, I evacuate the mold by means of one of the stop cocks 40. An evacuation down to the vapor pressure of the monomer mixture will be sufllcient. The vacuum naturally tends to hold the end plates 36 and 31 in place and thus helps to make a tight seal. Furthermore, in case a silver wire gasket is used, it is preferable to have a vacuum holding the end plates in position for then the pressure is uniform and not localized at the bolts and this sets up fewer strains in the metal end plates.

The mold in this example is 25 centimeters in diameter and 75 centimeters long and contains enough polymer to make a sheet one centimeter thick. In this case the mold i placed in the oven with its axis horizontal and is rotated at a speed of 600 revolutions per minute. At that speed the monomeric mixture spreads out evenly on the cylindrical surface.

The oven is now heated to a temperature of 65 C. and the apparatus is maintained in the oven for-eight to ten hours. This causes po merization of the liquid into a solid, clear, transparent resin of cylindrical shape which can then be removed from the mold and out, as already described. The cut cylindrical polymer is then flattened out between polished chromium plated press plates heated to a temperature of 160' 0., holding the sheets between the plates for about five minutes. The resulting polymeric sheet is found to be uniformthroughout and to have very good surfaces. The process can also be carried out at 60' C. if the time is increased to from twenty to twenty-four hours.

This polymeric sheet is remarkably colorless. m the manufacture of this resin heretofore one of the major diiliculties encountered been the formation of a yellow color. The cause of the color has been very elusive. It has been variously attributed to gasket material, to delay in polymerizing, to impurities in reagents, and to the presence of minute amounts of inhibitors. Each of these possible causes of color has been shown to be capable of causing color, but the appeare ance of color has persisted even when extreme care was taken to eliminate these sources and the color has been variable in intensity from batch to batch.

By proceeding as in Example I, however, all color is substantially eliminated. I attribute this to a small amount of peroxide used. I find that in general too much peroxide catalyst has previously been used in polymerizing unsaturated methylene compounds. I believe that the peroxides of the unsaturated methylene compounds, equally with the "foreign" peroxides such as henzoyl peroxide, break down and cause this yellow color. When proceeding according to Example I, however. the minute amount of peroxide used practically eliminates this defect. 'For example. a piece can be made which i '15 centimeters long, and viewed through the greatest dimension scarcely a trace of yellow is visible to the eye.

Example II Proceeding as in Example I, I use, however, a mixture of monomeric methyl methacrylate and 10% monomeric methacrylic anhydride. The monomeric mixture is oxygen-free but has .005% peroxide calculated as benzoyl peroxide. This is rotated at the same peed and kept in the oven for the same length of time and treated in exactly the same way. The results are also the same excepting, of course, .that the substance is a different substance.

After hardening for eight to ten hours at a temperature of 130' C., the sheet of Example I has a on the Rockwell C scale at 28" C. using 60 kg. load on a '16" ball of 70 and the sheet of Example II has a hardness on the Rockwell scale at the same temperature oi 65. The sheets of both examples will have a tensile strength of at least 1000 pounds per square inch at 100 C.

By the use of the method of the invention puckerlng and bubbling are avoided. It has been known that methyl methacrylate and interpolymers thereof are diillcult to polymerize because they contract on so doing and draw away from the sides of any mold equipment used unless special precautions are taken. It has also been found difllcult to make a good product in an open mold although the reason for this was not particularly known. One reason is that oxygen i an inhibitor for the polymerization. My method readily lends itself to use of an apparatus by ascasao cock 40 to let out the air and the other stop cock ll to admit a gas. Suitable gases are the inert gases, such as argon and helium and also gases like hydrogen and carbon dioxide and nitrogen which, while not inert, are non-oxidising.

In fact, most non-oxidising gases maybe used quite successfully.

There are special advantages in centrifugally casting the polymer while excluding oxygen therefrom. In the first place, centrifugal casting as herein disclosed eliminates any necessity to make allowances for contraction of the material as it polymerizes. For the action is one of increasing viscosity and with the great pressure due to centrifuging there are no bubbles or cracks or news termed and the surfaces are very regular and smooth. But centrifugal casting in the presence oi air will not give the best results because with such a large surface exposed to air with both the air and the liquid in a state of agitation, much oxygen would get into the monomer and greatly retard polymerization. My experiments indicate that in a rotating mold filled with air no first grade polymeric sheets from monorneric unsaturated methylene compounds can be made.

As a result of my experiment and in accordance with the foregoing theories, I have determined that it is not necessary to use such large amounts of peroxide catalyst as have customarily been used in the past. I have further determined that excessive amounts of catalyst are a positive disadvantage. From practical and commercial considerations a great deal depends upon the requirements as to how much discoloration can be tolerated. So far a certain ieatures of my invention are concerned. superior results are obtained by polvmeriaing with heat in the ab sence of oxygen even though there is as much catalyst present as 1% calculated as benzoyl peroxide. On the other hand, there is no advantage that I know of in using so much peroxide catalyst and less than 1%, b weight of the monomeric compound, of peroxide catalyst will ordinarlly be used according to my invention. The preferred amount is somewhere around .005% calculated as benzoyl peroxide. I mayreadily use lesser amount; than this and preferably I use no more than .01% calculated as benzoyl peroxide.

One of the featuru of the invention is that by using a small amount of catalyst and excluding oxygen there is less exothermic heat of reaction to be controlled. This simplifies the practical problems of producing polymers in commercial quantitie and renders it possible to use simpler apparatus.

Another advantage of the invention is that I can polymerize at a low temperature and thereby obtain a product of higher molecular weight.

If a given monomer is polymerized at two different temperatures. generally speaking the lot which is polymerized at the lower temperature will have a higher molecular weight. This appears to be due to a chain stopping reaction which takes place more readily at a higher temperature. when polymers were made in the presence of air certain temperatures were used, but I am able to use for these same polymers a lower temperature due to the absence of oiwgen. Therefore, I am able to obtain higher molecular weight polymers.

assasao Example III Taking a quantity of methyl methacrylate monomer I formed therein peroxide equivalent to .005% benzoyl peroxide. I placed it in a suitable mold or container from which oxygen wa excluded. I heated the monomer to 50 C. and maintained it at this temperature for about two days. The resulting polymeric methyl methacrylate was found to be completely insoluble in benzene. Polymeric methyl methacrylate as previously made by heating the monomer has been readil soluble in benzene. This lot was solid within a few hour and the additional time was a precaution to be sure that the polymerization was complete. Actually it was probably complete in a much shorter time than two days.

It will be seen that according to the invention polymerization is accelerated. It is also apparent that I can make a polymer 01' higher molecular weight and a tougher product than ha heretofore been made by previous methods involving heat. From many practical considerations it is preferred to cause polymerization by heat rather than light, so the invention finds practical utility in commercial production of polymers. Another ieature of the invention is that results can be duplicated day after day, year after year, something which has been found hard to do heretofore. Furthermore, when it is desired to use resins ior optical purposes or for windows for vehicles and otherwise, it is highly desirable that they be as colorless as possible. I have found that the presence of a significant amount or oxygen during polymerization or eve after polymerization causes discoloration of the product, usually yellowing it. According to the invention a very colorless product can be iormed.

So far as the method of the invention is concerned the oxygen can be excluded in other ways than removing it from the chamber in which the monomer is polymerized. For example, if a container is almost completely filled with the monomeric material and then sealed up, the air which is left therein will react with the monomer to form peroxide and this reaction will use up the oxygen. So long a the container is tightly sealed and no more oxygen can get into it, the polymerization may then be said to go forward in the presence of peroxide and the absence of oxygen. Within the scope of this invention the method may be carried out in this manner.

Relatively pure peroxide of methyl methacrylate or of any of the other unsaturated methylene compounds herein mentioned may be formed by taking the monomeric material and heating it in the presence or oxygen (not air). The reaction takes place more rapidly if pressure i used. Instead of heat, ultra-violet light may be used. To shorten the period needed for preparing the pure peroxide, after the reaction has proceeded for a considerable time, the balance of the monomer may be evaporated under reduced pressure. Thus is formed a pure or relatively pure peroxide of methyl methacrylate, or of methacrylic acid, or of vinyl acetate. These peroxides are found to be colorless oily liquids. When heated they suddenly explode with dangerous violence. They also liberate iodine from potassium iodide very readily and this is a methd of identification.

It will thus be seen that there has been provided by this invention a method and apparatus in which the various objects hereinabove set forth together with many thoroughly practical advantages are successfully achieved. As various possible embodiments might be made of the mechanical features 01' the above invention and a the art herein described might be varied in various parts, all without departing from the scope of the invention, it is to be understood that all matter hereinbetore set forth or shown in the accompanying drawings is to be interpreted as illustrative and not in a limiting sense.

I claim:

1. Method of polymerizing a monomeric compound irom a group consisting of lower alkyl esters of acrylic acid and methacrylic acid to form a solid object which comprises removing dissolved oxygen from said monomeric compound and subsequently heating said monomeric compound in a mold in an oxygen-free state and in the presence of less than 1% by weight of said monomeric compound of peroxide catalyst calculated as benzoyl peroxide to convert said monomeric compound to the polymer while excluding any appreciable amount of oxygen from contact with said monomeric compound.

2. Method of polymerizing a monomeric compound from a group consisting of lower alkyl esters of acrylic acid and methacrylic acid to form a solid object which comprises removing dissolved oxygen irom said monomeric compound and subsequently heating said monomeric compound in a mold in an oxygen-free state and in the presence of between .005% and 0.01% by weight of said monomeric compound of peroxide catalyst calculated as benzoyl peroxide to convert said monomeric compound to the polymer while excluding any appreciable amount of oxygen from contact with said monomeric compound.

3. Method oi polymerizing monomeric methyl methacrylate to form a solid object which comprises removing dissolved oxygen from said monomeric methyl methacrylate and subsequently heating said monomeric methyl methacrylate in a mold in an oxygen-tree state and in the presence of less than 1% by weight of said monomeric methyl methacrylate of peroxide catalyst calculated as benzoyl peroxide to convert said monomeric methyl methacrylate to the polymer while excluding any appreciable amount of oxygen from contact with said monomeric methyl methacrylate.

4. Method of polymerizing monomeric methyl methacrylate to form a solid object which oomprises removing dissolved oxygen from said monomeric methyl methacrylate and subsequently heating said monomeric methyl methacrylate in a mold in an oxygen-free state and in the presence of between .005% and 0.01% by weight of said monomeric methyl methacrylate of peroxide catalyst calculated as benzoyl peroxide to convert said monomeric methyl methacrylate to the polymer while excluding any appreciable amount of oxygen from contact with said monomeric methyl methacrylate.

5. The method of making a synthetic resin comprising the steps of providing a monomeric compound from the group consisting of the lower alkyl esters of acrylic acid and methacrylic acid, developing its peroxide content by treating said monomeric compound with oxygen under the infiuence of heat and forming a peroxide content therein suflicient to catalyze the polymerization of said monomeric compound but less than 0.01 per cent by weight of said monomeric compound, of peroxide catalyst calculated as benzoyl peroxide. removing any dissolved oxygen remaining in 8 s,see,eso

said monomeric compound, and theneaiter heatin: said monomeric compound to convert same to polymer, while excluding any appreciable amount of oxygen from contact with said monomeric compound.

6. Method of forming a polymer from a monomeric compound from the group consisting of the lower alkyl esters oi acrylic acid and methaerylic acid, which comprises removing dissolved oxygen from said monomeric compound and subsequently o polymerizing said monomeric compound in the presence of less than 1%, by weiaht 0! said monomeric compound, oi peroxide catalyst calculated as bensoyl peroxide, while excluding any appremethacrylate.

CARL E. BARNES.

CERTIQI CATE 0F QORREQTION Patent No. 2,369,520.

February 15, 1914.5.

CARL E. BARNES.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 2, seconci column, line 35, for that portion of the formula reading "N0 read "H0 and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Sigied arzi sealed this 5th day of June, A. D. 19LL5.

(Seal) Leslie Frazer Acting- Commissioner of Patents.

8 s,see,eso

said monomeric compound, and theneaiter heatin: said monomeric compound to convert same to polymer, while excluding any appreciable amount of oxygen from contact with said monomeric compound.

6. Method of forming a polymer from a monomeric compound from the group consisting of the lower alkyl esters oi acrylic acid and methaerylic acid, which comprises removing dissolved oxygen from said monomeric compound and subsequently o polymerizing said monomeric compound in the presence of less than 1%, by weiaht 0! said monomeric compound, oi peroxide catalyst calculated as bensoyl peroxide, while excluding any appremethacrylate.

CARL E. BARNES.

CERTIQI CATE 0F QORREQTION Patent No. 2,369,520.

February 15, 1914.5.

CARL E. BARNES.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 2, seconci column, line 35, for that portion of the formula reading "N0 read "H0 and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Office.

Sigied arzi sealed this 5th day of June, A. D. 19LL5.

(Seal) Leslie Frazer Acting- Commissioner of Patents. 

